Understanding the unintended spread and impact of alien and invasive fish species – development of management guidelines for South African inland waters

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South African rivers contain several alien invasive predatory fish species that are known to cause ecological harm, and many of these species are continuing to expand their ranges. As a result of the recently promulgated regulations under the National Environmental Management: Biodiversity Act 10 of 2004, South Africans now have a legal responsibility to prevent the spread of alien fish into areas and ecosystems where they are likely to cause environmental harm.
This study has produced a decision support tool as a framework for eradication and management of invasive fish in South African river systems, in addition to adding to our understanding of the spread and impacts of alien invasive fishes in South African Aquatic ecosystems. The decision support tool was utilised in two case studies which, represented the most common issues faced by conservation bodies in the management of the impacts of alien invasive fishes. These are: Conservation of indigenous and management of invasive alien fishes in the headwaters of the Groot Marico River (Northwest Province) and understanding and mitigating the unintentional spread of invasive fish through the Sundays River Irrigation Scheme (Eastern Cape Province). Two additional case studies were completed on Nseleni and Swartkops rivers.
In the Sundays example, the study demonstrated that while alien fish populations within the irrigation network itself are unmanageable, the invasive catfish population in the Coerney River can be managed, although eradication is deemed not feasible within the stream. The study investigated three discreet populations of bass in the Groot Marico catchment. The endemic Marico barb (Barbus motebensis) was the only species that was strongly negatively correlated with the presence of M. salmoides. The decision tree indicated that only the population in the Marico Eye is an appropriate target for eradication, due to its small geographic extent and its proximity to a vulnerable stream reach (the Kaaloog se Loop stream) that could easily be invaded as a result of human activity. Four tributary populations were identified as conservation priority areas, namely Draaifontein, Van Straatens, Ribbokfontein and Kaaloog se Loop tributaries, where it is critical that management strategies be set. The study also demonstrated that the Loricariid catfish in the Nseleni river and Centrarchid basses in the Swartkops river cannot be eradicated for logistical reasons.
Developing strategies to mitigate the spread of alien invasive fishes, in order to effectively conserve indigenous biota and manage the impact of alien invasive fish species, is therefore important. To be effective, strategies must be based on an understanding of the distribution, biology, ecology and impact of both the alien and the indigenous fishes, particularly in rivers of high conservation importance. In addition, since invasive species can spread between water bodies with the unintentional aid of humans, understanding all human-mediated introduction pathways, is crucial for developing management strategies, that can mitigate further ecosystem damage through fish invasions.